script maldito

[juliobejines]

Hola no consigo que funcione este script q solo suma valores...
<script>
function suma(){
a=document.formulario;
a.valor3.value=parseInt(a.select.value)+parseInt(a .select2.value)+parseInt(a.select3.value);
}
</script>


<form action"" name="formulario" method="post" id="formulario">
<label>opciones
<select name="select" id="select" onclick="suma()">
<option value="580">opcion a elegir 1</option>
<option value="1300">opcion a elegir 2</option>
<option value="600">opcion a elegir 3</option>
</select>
</label>
<p>
<label> PROVINCIA
<select name="select2" id="select2" onclick="suma()">
<option value="30">Huelva</option>
<option value="20" selected>Sevilla</option>
<option value="30">C&aacute;diz</option>
<option value="30">C&oacute;rdoba</option>
<option value="30">M&aacute;laga</option>
<option value="30">Ja&eacute;n</option>
<option value="40">Granada</option>
<option value="50">Almer&iacute;a</option>
</select>
</label>
</p>
<p>

<input type="checkbox" name="select3" id="select3" value="1000" onclick="suma()">
</p>
<p>
<label>
<input type="submit" name="Submit" value="Obtener presupuesto">
</label>
<input name="valor3" type="text" id="valor3">
</p>
</form>